The present disclosure relates to a thermal probe for a scanning thermal microscope, use of the thermal probe, and a process of manufacturing the thermal probe.
Local temperature and thermal conductivity of a sample interface can be measured e.g. using a scanning thermal microscope (SThM) with a thermal probe comprising a probe tip. Generally, when two bodies at different temperatures are separated by a gap, they can exchange heat via thermal radiation. In the far-field approximation, i.e. when the gap is much larger than the characteristic thermal wavelength, the magnitude of heat transfer may be described by the classic Stefan Boltzmann law. However, when the thermal probe is brought close to the sample interface, i.e. in the “near-field”, non-classical phenomena such as wave interference, surface resonances and photon tunnelling can become important, typically causing the radiative heat transfer to increase with decreasing gap size. Also conductive and/or convective heat transfer may become important at close proximity to the surface.
For example, Nakabeppu et al. (Appl. Phys. Lett., Vol. 66, No. 6, 1995) describes scanning thermal imaging microscopy using composite cantilever probes. The technique uses the atomic force microscope (AFM) to scan a composite (bi-material) cantilever probe made of a thin metal film deposited on a regular silicon nitride AFM probe. During tip-surface contact, heat flow through the tip changes the cantilever temperature which bends the cantilever due to differential thermal expansion of the two probe materials. However, micro-scale forces on the probe tip at close proximity, such as van der Waals forces and electrostatic forces, can cause interference with the thermal measurement.
For example, Shen et al. (Nano Lett. 9, 2909-13, 2009) describes surface phonon polaritons mediated energy transfer between nanoscale gaps using a microsphere that is glued to a bi-layer (bi-material) cantilever. When heat is transferred from the probe tip to the sample substrate or vice versa the temperature distribution over the cantilever changes, causing its material layers to expand. Because the materials have different heat expansion coefficients, the cantilever tends to curve in reaction to the heat that is transferred between the probe and the substrate. The curvature can be measured e.g. using a laser beam focused on the tip of the cantilever and reflected onto a position sensitive detector. However, it is difficult to control the close distance between the probe tip and sample interface since this is influenced by the heat exchange and interaction forces with the sample interface.
Accordingly, there is a need for an improved thermal probe for a scanning thermal microscope wherein disadvantages of the prior art are alleviated.